Traditionally, soap has been utilized as a skin cleanser. Notwithstanding its many advantages (e.g., inexpensive, easy to manufacture into bars, having good lathering properties), soap is a very harsh chemical. Irritated and cracked skin often result from the use of soap, especially in colder climates.
In order to maintain cleaning effectiveness and reduce harshness, the art has used synthetic surfactants to replace some or all of the soap. In particular, anionic surfactants have been used because these tend to most clearly mimic the lather generation which soap readily provides. Synthetic bars and soap-based bars have significantly different processing and user properties; for example, synthetic bars often require a structurant or binder while soap-based bars do not.
Anionic surfactants, however, are still harsh. One method of reducing the harshness of anionic surfactants is to utilize other surfactants such as nonionic or other mildness surfactants (e.g., amphoteric). The use of surfactants other than anionics, however, can introduce other problems. For example, nonionic surfactants generally do not generate creamy thick lather as do anionics; and both nonionics and amphoterics, for example can be sticky and introduce processing difficulties.
For this reason, the art is always searching for materials which are milder than anionic and/or which can be used to replace at least some of the anionic surfactants, yet, which do not simultaneously seriously compromise lather generation or processing efficiency. Further, even if the anionic is not substituted, the art is always searching for materials which can substitute for inerts and/or other fillers and produce enhanced mildness.
Unexpectedly, applicants have found that these goals can be obtained by inclusion of relatively low levels of specific nonionic surfactants in specific synthetic bar compositions (i.e., structured at least partially by polyalkylene glycol). That is, at anionic to nonionic surfactant weight ratio between 1:1 to 10:1, the nonionic surfactants provide significantly enhanced mildness without sacrificing processability or lather. While not wishing to be bound by theory, it is believed that the nonionic surfactants may be interacting with anionic surfactants to form mixed-micelle type of colloidal complexes thereby reducing free anionic surfactant (known for its harshness) from the bar.
The use of alkoxylated nonionic surfactants in bar compositions per se is not new. Prior art has shown that addition of these nonionics in fatty acid soap based bars can reduce scum formation and reduce skin irritation by reducing soap residue on skin after washing in hard water. Nonionic surfactants have also been used as cosurfactants and as solvents for antibacterial agents in soap bars. They have also been used as detergents in synthetic bars in general.
World Patent No. WO 9,317,088 to Procter & Gamble, for example, teaches a soap-based bar comprising 45-90% fatty acid soap, 1-8% nonionic C.sub.14-20 EO.sub.65-100 as coactive (EO: ethylene oxide), and 0.5-2% cationic polymer as mildness aid.
World Patent No. WO 9,304,161 to Procter & Gamble teaches a soap-based bar comprising 45-90% fatty acid soap, 0.5-10% C.sub.14-20 EO.sub.20-250 (preferably C.sub.14-20 EO.sub.25-80) as cosurfactant, and 0.5-10% acyl isethionate surfactant. The purpose of addition of small amounts of alkoxylated nonionic surfactants was to reduce the scum formation.
Patent No. GB 2,243,615 to J. Dunbar, R. Bartolo, B. Redd, and A. Keegan teaches an antibacterial toilet soap bar containing 45-94% alkali metal soap (at least 50% in Beta-phase), 2-25% solvent for antibacterial agents, 0-30% non-solvent synthetic detergents, and 0-10% fatty acid. The solvent for antibacterial agents is selected from polyethylene glycol and nonionic alkoxylated fatty alcohols in general.
Patent No. EP 311,343 to G. Dawson and G. Ridley teaches a Beta-phase toilet soap bar comprising 45-90% of soluble alkali metal soap of C8-C24 fatty acids, 0.5-45% of an alkoxylated nonionic surfactant having an HLB of 12-19.5, and 0.01 to 5% of a water-soluble polymer. The composition has improved scum control with good mildness, lathering, and transparency.
Patent No. EP 363,215 to F. Simion, R. Subramanya, R. Cantore, and D. Masucci teaches an ultra-mild skin cleansing bar comprising 25-90% (preferably 65-95%) fatty acid soap and 5-75% (preferably 5-35%) alkoxylated nonionic or anionic surfactants (C.sub.8 E.sub.j&gt;3 A, A=OH or anionic head groups). The soap bar is claimed to be very mild and reduce skin irritation by reducing soap residue left after washing in hard water.
Patent No. EP 213,729 to A. Hight teaches a soap bar containing 5-50% fatty acid soap, 5-25% alkoxylated nonionic detergent as coactive, and 0-10% phosphate builder. High levels of soap were included in the bar composition (weight ratio of fatty acid soap to ethoxylates is 1:1 to 10:1).
Patent No. EP 287,300 to C. Adam, G. Irlam, and R. Lee teaches a soap bar made by high energy shear at low temp. (&lt;0.40 C) comprising 20-80% fatty acid soap, 10-60% non-soap detergent that is selected from C8-C18 anionic surfactants and nonionic surfactants, such as alkoxylated alcohols in general.
Patent No. GB 2,276,630 to P. Powers teaches a laundry detergent bar contains 10-60% non-soap anionic detergent (at least 10% alkylbenzene sulphonates and alkyl sulphates), 5-60% detergent builder and 0.3-4% alkoxylated nonionic detergent. The bar gives reduced mush when left standing in water.
Patent No. EP 507,559 to S. Pratley teaches a cast-melting bar comprising 25-60% anionic, zwitterionic and nonionic (i.e. alkoxylated nonionic) surfactants in which 8-32% are fatty acid soap. Also 10-50% alcohols are included as solvents, and 1-20% of an oily skin benefit agent is included.
U.S. Pat. Nos. 3,312,626 and 3,312, 627 to D. Hooker teaches a nonionic bar composition substantially free of anionic surfactants containing 10-70% nonionic detergents, in which alkoxylated nonionic surfactants are among the candidates. The bar also contains 0-70% PEG, EO-PO and derivatives of these compounds as structurant. In order to give these bars more "soap-like" characteristics, the reference contemplates use of 10%-80% lithium soap. It is clear that use of lithium soap is unique to the invention (column 8, lines 20-23) and that use of other soaps or anionic (other than fatty acid lithium soap) is not contemplated.
The subject invention differs from the prior art referred above, alone or in combination, in that the applicants have found that relatively low levels of specific alkoxylated nonionic surfactants (i.e., having specific molecular weight, specific melting temperature, and specific hydrophilic to hydrophobic mol ratio) most effectively mitigate the skin irritation of anionic surfactants of a personal washing bar which comprise 10 to 70% of a surfactant system of which at least 50% (though no more than 40% total of total composition) is synthetic anionic surfactant.
Also novel to the art, our invention incorporated these low levels of specific alkoxylated nonionic surfactants into specific synthetic bar compositions (i.e., structured and binded at least partially by polyalkylene glycol or derivatives of polyalkylene glycol, such as EO-PO copolymer and other hydrophobically modified polyalkylene glycol) without sacrificing processability, biodegradability, and desired user properties, such as lather, bar smoothness and homogeneity.